The invention relates to optical amplifiers and light sources. By way of example, though not exclusively, the invention relates to single- or few-moded waveguiding lasers, superfluorescent sources, optical amplifiers, high pulse-energy devices, energy-storage devices, cladding-pumped devices, semiconductor signal amplifiers, and waveguiding saturable absorbers.
The tightly confined modal fields of single- or few-moded waveguiding lasers, superfluorescent sources, and amplifiers lead to a very strong interaction between any waveguided light and the active medium in the waveguiding core. Therefore, a comparatively small amount of gain medium is sufficient for providing the gain in these devices. Specifically, the gain for a given stored energy, as well as for a given absorbed pump power, is high. This is often beneficial, since it means that the pump power requirements for a given desired laser output power or amplifier gain can be low.
However, for several devices, this efficient interaction between mode and gain medium can be detrimental. The following example refers to certain types of amplifiers and lasers, but of course the skilled man will realise that the same or similar problems can occur in, for example, superfluorescent sources.
In a laser or amplifier, the achievable single-pass gain is limited to, say, 50 dB. The reason is that at this gain, a significant fraction of the pump power is converted to amplified spontaneous emission (ASE). A 10 dB higher gain results in approximately 10 dB more ASE, so at these gains, the extra pump power required to increase the gain further will be prohibitively high. Since the ASE limits the gain of the device, it also limits the energy stored in the gain media. This in turn obviously limits the amount of energy that a pulse can extract from the device. Consequently, the pulse energy that can be obtained from waveguiding lasers and amplifiers is limited. Instead, bulk (i.e., not waveguiding) lasers and amplifiers for which the extractable energy for a given gain can be several orders of magnitude lower are often employed to provide much higher pulse energies. However, the robustness and stability of bulk lasers is often inferior to waveguiding ones.
Moreover, the gain limit can also be problematic for lasers and amplifiers irrespective of whether the stored energy is a major concern, if the high gain appears at another wavelength than the desired one. The reason is that ASE (or lasing) at the gain peak will suppress the gain achievable at the desired wavelength, possibly to a value below what is required for a good amplifier or laser. This applies to all types of amplifiers and lasers. Furthermore, in optically pumped lasers and amplifiers, a suitable interaction between the gain medium and the amplified or generated signal beam is not enough; also the interaction between the pump beam and the gain medium must be appropriate. However, in some types of lasers and amplifiers (typically cladding-pumped ones), the interaction with the pump beam is significantly smaller than the interaction with the signal beam. Then, for a device that efficiently absorbs the pump, the interaction with the signal beam will be much stronger than what is required. Unfortunately, this excess interaction is often accompanied by excess losses for the signal beam, since:
1. The scattering loss of an active medium is normally higher than it can be for a passive medium. For instance, rare-earth-doped fibers have scattering losses of, e.g., several orders of magnitude higher than standard, passive, single-mode fibers.
2. A fraction of the active medium often has inferior properties. For instance, in Er-doped fibers, pairs of Er3+-ions can form. These result in an unbleachable loss. The strong interaction then leads to a high loss.
3. The active medium in its amplifying state can also absorb light (so-called excited-state absorption, ESA). Again, a stronger interaction leads to more power lost through ESA.
Moreover, a bleachable medium (e.g., an unpumped gain medium with a ground-state absorption) can be used as a saturable absorber. An efficient interaction leads to a low saturation power. A reduced interaction leads to a higher saturation power, which can be more suitable for some applications, especially if the interaction, and hence the saturation power, can be controlled.
Clearly, although often beneficial, the tight confinement of the guided light is a problem for some devices.
An aim of the present invention is to improve the interaction between light guided along a waveguide and rare-earth dopants within an active medium.
Accordingly in one non-limiting embodiment of the present invention, there is provided apparatus comprising a waveguide and an amplifying region wherein the waveguide comprises a core and a cladding and the amplifying region comprises rare-earth dopants and wherein the amplifying region comprises a ring around the core of the waveguide.
Various aspects of the invention are defined in the appended claims, and in passages throughout the present application.
According to a first embodiment of the present invention, there is provided an amplifying optical device comprising a first waveguiding structure comprising a first core and cladding and configured to guide optical radiation, at least one pump source configured to supply optical pump power, an amplifying region situated in the cladding; and wherein the pump source is optically coupled to the amplifying region; and wherein in use the optical radiation guided in the first waveguiding structure overlaps the amplifying region.
The invention also provides a method of pumping at least one optical fiber amplifier with a fiber laser, the method comprising providing a first waveguiding structure fabricated from at least one glass system and comprising a first core and cladding; providing a second waveguiding structure comprising a second core at least partly formed by the cladding and an amplifying region comprising Ytterbium; providing a source of optical pump power in optical communication with the second waveguiding structure and having a wavelength in the band from about 870 nm to about 950 nm; providing an optical feedback device; guiding optical radiation using the first waveguiding structure; guiding the optical pump power using the second waveguiding structure such that the amplifying region interacts with the optical radiation guided in the first waveguiding structure and the optical pump power guided in the second waveguiding structure to amplify the optical radiation guided by the first waveguiding structure; using the optical feedback device to ensure that a plurality of times a portion of the optical radiation guided by the first waveguiding structure is amplified more than once by the amplifying region; providing an amplifying region characterized by a dopant concentration, a disposition and a length, and wherein the dopant concentration, the disposition and the length of the amplifying region are arranged such that the fiber laser emits optical radiation at an emission wavelength in the region of about 970 nm to about 990 nm; and coupling the optical radiation at the emission wavelength in the region of about 970 nm to 990 nm into the at least one optical amplifier.
A second method provided by the invention is a method of amplifying optical pulses to energies exceeding the intrinsic saturation energy of an amplifying optical device, comprising: providing a first waveguiding structure comprising a first core and cladding; providing a source of optical pump power; providing a second waveguiding structure comprising a second core at least partly formed by at least part of the cladding, and an amplifying region; guiding optical radiation using the first waveguiding structure; and guiding the optical pump power using the second waveguiding structure such that the amplifying region interacts with the optical radiation guided in the first waveguiding structure and the optical pump power guided in the second waveguiding structure.
A third method provided by the invention is the method of using a waveguiding saturating absorber comprising: providing a waveguiding structure having a core and a cladding; guiding optical radiation in the waveguiding structure; providing an absorbing region situated within the cladding and disposed such that it provides an absorption of the optical radiation guided in the core such that in use at least 10% of the absorption is bleached by the optical radiation guided by the core in at least a part of the waveguiding saturating absorber at least part of the time.
According to a second embodiment of the present invention, there is provided an amplifying optical device comprising: a first waveguiding structure configured to guide optical radiation which can propagate in a fundamental mode; a pump source configured to supply optical pump power; and a second waveguiding structure configured to guide the optical pump power, wherein the pump source is optically coupled to the second waveguiding structure; and wherein in use the optical radiation is characterized by an optical power distribution of the fundamental mode having a contour of equal intensity perpendicular to the local longitudinal axis of the first waveguiding structure the contour enclosing about 75% of the optical power of the fundamental mode; and wherein the second waveguiding structure contains an amplifying region situated to interact with the optical pump power guided in the second waveguiding structure when the amplifying optical device is in use; and wherein the amplifying region is situated to lie outside the contour of equal intensity; and wherein during use at least 0.1% of the optical radiation guided by the first waveguiding structure overlaps the amplifying region.
Embodiments of the invention provide devices that are considerably improved by a predetermined reduction of the interaction between a signal light beam and an active medium (per unit volume) compared to prior-art designs, without necessarily changing the properties of the gain medium or reducing the confinement of the signal light (although a reduced confinement can also be beneficial for the disclosed devices). The active medium serves to amplify or generate the signal light beam, or, if unpumped, can act as a saturable absorber.
The reduction in interaction is achieved by placing the bulk of the active medium in regions where the intensity of the signal beam is substantially smaller than its peak intensity, in a cross-section of the waveguiding device perpendicular to the direction of propagation of the signal beam. This can provide advantages for the following devices:
1. Lasers (e.g., Q-switched and gain-switched ones) and amplifiers in which it is desirable to store large energies. In these devices (as well as for so-called energy-storage devices in general), the reduced interaction leads to a larger stored energy before practical upper limits on the gain is reached.
2. Optical amplifiers (typically semiconductor ones) for which even the energy of a single signal bit can be comparable to the stored energy. In those, already the amplification of a bit extracts enough energy to reduce the gain. This leads to four-wave mixing, cross-talk, and inter-symbol interference. This can be reduced with the higher stored energy that, for a given gain, accompanies the reduced interaction.
3. Amplifiers and lasers in which an efficient pump absorption necessitates large amounts of gain media, which in prior-art devices leads to excessive small-signal absorption, background absorption, or excited state absorption at the operating wavelength, or excessive gain at another wavelength. A reduced interaction then leads to reduced losses. Moreover, a reduced interaction can reduce the gain at the undesired wavelength relative to that at the desired one, and thereby the problems associated with a too high gain at the wrong wavelength. This applies to lasers in which there is a significant unpumped loss (typically, reabsorption loss or out-coupling loss). These points are especially relevant for cladding-pumped devices. For example, to ensure sufficient pump absorption, the fiber may need to be so long that one or both of those problems arise.
4. Saturable absorbers, in which the saturation power is otherwise too small.
Embodiments of the invention can overcome or alleviate some of the problems described above and can at least partially achieve one or more of the following:
1. To reduce the susceptibility to so-called quenching and background losses, in particular for cladding-pumped devices.
2. To obtain efficient emission at wavelengths otherwise inaccessible for devices where there is a significant unpumped loss, in particular for cladding-pumped devices.
3. To improve the energy storage capabilities, for energy-storage devices.
4. To reduce signal cross-talk and inter-symbol interference for signal amplifiers.
5. To allow for a larger, predetermined, saturation power.
Embodiments of the invention can provide the following devices and embodiments, and the use of the following amplifying and/or absorbing waveguiding structures in such devices:
1. An amplifying optical fiber in which the active medium is placed partly or wholly outside the waveguiding core, e.g., in a ring around the core. The gain medium can also reside inside the core in regions where the normalized modal intensity of the signal beam is small. The fiber can be made of a glass, partly doped with Pr3+, Tm3+, Sm3+, Ho3+, Nd3+, Er3+, or Yb3+, or a combination thereof, and it can be cladding-pumped.
2. A cladding-pumped amplifier or laser in which the difference between the overlaps of the pump and signal beams with gain medium is substantially reduced compared to prior-art designs.
3. A ring-doped optical fiber for high-energy pulse amplification or generation or other energy storage applications. The fiber can for instance be made of a glass, partly doped with Pr3+, Tm3+, Sm3+, Ho3+, Nd3+, Er3+, or Yb3+, or a combination thereof, and it can be cladding-pumped. Moreover, the device can incorporate a longitudinally distributed saturable absorber to suppress the build-up of ASE. In one embodiment, the gain medium is a Yb3+-sensitized Er3+-doped glass, and the saturable absorber is an Er3+-doped glass, and they are located so that the signal intensity is higher in the saturable absorber than in the gain medium.
4. A Q-switched or gain-switched fiber laser based on an amplifying fiber with a relatively higher saturation energy combined with a saturable absorber fiber having a relatively lower saturation energy. The difference in saturation energy stems, at least to a significant part, from differences in the geometry of the fibers. The active media in the different fibers can be the same or different, and can for instance be a glass doped with a rare earth, e.g., Pr3+, Tm3+, Sm3+, Ho3+, Nd3+, Er3+, or Yb3+, or a combination thereof.
5. A ring-doped, cladding-pumped ytterbium-doped fiber for amplification or generation of light in the range 950 nm to 1050 nm.
6. A ring-doped, cladding-pumped neodymium-doped fiber for amplification or generation of light in the range 850 nm to 950 nm.
7. A ring-doped, cladding-pumped erbium-doped fiber for amplification or generation of light in the range 1450 nm to 1600 nm.
8. An amplifying planar waveguide structure in which the active medium is placed partly or wholly outside the waveguiding core, thus interacting with the signal beam only where the normalized intensity of the modal field is small. The waveguide can be cladding-pumped. Moreover, the design can be specifically adapted to correspond to any of the fiber devices listed above.
9. A semiconductor amplifier for signal amplification, in which the gain region is placed partly or wholly outside the waveguiding core, thus interacting with the signal beams only where their normalized modal intensities are small. Thereby, the saturation energy of the device will be increased, which subsequently reduces the inter-symbol interference and inter-wavelength cross-talk.
10. A waveguiding structure with a saturable absorption, in which the absorbing medium is placed partly or wholly outside the waveguiding core, thus interacting with the signal beam only where its normalized modal intensity is small.
Evanescent-field devices, including ring-doped fiber devices have not been considered for devices of the type proposed here, nor has any device been proposed or demonstrated based on ring-doping (or evanescent field interaction) that provide significant benefits of the type considered here, compared to traditional devices in which the gain-medium resides in the core in places where the interaction with the signal beam is large. Specific differences between embodiments of the invention and a prior art device are as follows:
1. It has not been one of the specific devices considered here.
2. It has not used a single-moded or few-moded waveguiding core.
3. It has not been a device in which the energy extraction results in cross-talk or inter-symbol interference.
4. The control of the emission wavelength that we propose has not been obtained.
5. The device has not substantially reduced the effect of losses at the signal wavelength.
6. It has not been a cladding-pumped device.
7. It has not been a device for high-energy pulses.
8. It has not been an optical fiber doped with erbium or another rare-earth for high-energy pulses.
9. The output of the device could not be launched into a standard single-mode fiber through splicing or butt-coupling, nor has the device allowed for an easy launch of signal light.
10. The output beam has not been tightly confined.
11. It has been a device doped in regions of the core where the modal intensity is large.
12. It has been a device doped in a large area around the core (e.g., homogeneously in the cladding), hence rendering it inefficient for cladding-pumping.
13. It has not been a fiber structure, or at least not an all-fiber structure.
14. It has not been a solid-state device.
15. The interaction length has been limited to a few centimeters.
16. It has not been a high-gain device.
17. It has not been a device pumped by an optical beam guided along the amplifying medium.
18. It has not been possible to manufacture the device with standard manufacturing techniques for rare-earth doped fibers like MCVD and solution doping.
19. The purpose of the design has not been to obtain a smaller interaction between the gain medium and the signal light than would otherwise be possible, nor have any substantial benefits of a substantially smaller interaction been proposed, discussed, or demonstrated.